1. Field of the Invention
The present invention relates to an optical circuit used in two-way optical CATV or optical data transmission and a light transmission system using the same.
2. Related Art of the Invention
In the light transmission system and method, recently, as the information is sophisticated and diversified, transmission of larger capacity is demanded, and request type transmission is being realized, which gives rise to necessity of two-way exchange of video information, data, etc. Where multiple branching and distributing should be required as in optical CATV, it is necessary to compensate for branching loss by disposing optical amplifiers before and after branching. In the case of analog signal transmission, in order to eliminate the effects of return light, it is necessary to insert a optical isolator in at least one of the front and rear optical amplifiers.
Some examples of conventional light transmission system are described by referring to FIG. 23 to FIG. 25. Same reference numerals in the diagrams refer to identical members.
First, in FIG. 23, reference numeral 30 is a light source, 31 is a light receiver, 23 is a optical amplifier, 27 is a band pass filter passing only a signal light emitted from the light source 30, and 28 is a optical isolator for passing the light only in an arrow direction and blocking the return light advancing in reverse direction. Herein, the signal light emitted from the light source 30 is amplified by the optical amplifier 23, and the spectrum of the amplified light contains asynchronous spontaneous emission (ASE) light components possessed by the optical amplifier 23, and these components distribute in a wide wavelength region outside the wavelength region of the signal light. The band pass filter 27 removes the undesired ASE light components, which is beneficial for low noise transmission.
In FIG. 24, next, reference numerals 30a and 30b are light sources for sending light in wavelength .lambda..sub.1 and .lambda..sub.2, and 31a and 31b are light receivers for receiving the light in wavelength .lambda..sub.1 and .lambda..sub.2, and the light source 30a and light receiver 31a, and the light source 30b and light receiver 31b respectively form pairs. Reference numerals 21a, 21b, 21c, 21d, 21e, 21f are optical multi/demodulators for combining or separating light, 22a and 22b are optical isolators, and 23 is a optical amplifier.
The light in wavelength .lambda..sub.1 is emitted from the light source 30a, and is led into the optical multi/demodulator 21f through the optical multi/demodulator 21d and optical multi/demodulator 21c. On the other hand, the light in wavelength .lambda..sub.2 is emitted from the light source 30b and is led into the optical multi/demodulator 21f through the optical multi/demodulator 21a and optical multi/demodulator 21b and is combined with the light in wavelength .lambda..sub.1. The light multiplexed in wavelength in this way is amplified in batch in the optical amplifier 23 through the optical isolator 22b, and then separated in wavelength into light in wavelength .lambda..sub.1 and light in .lambda..sub.2 by the optical multi/demodulator 21e through the optical isolator 22a. The light in wavelength .lambda..sub.1 is led into the light receiver 31a through the optical multi/demodulator 21b and optical multi/demodulator 21a. On the other hand, the light in wavelength .lambda..sub.2 is led into the light receiver 31b through the optical multi/demodulator 21c and optical multi/demodulator 21d. By such operation, two-way transmission by light in wavelength .lambda..sub.1 and .lambda..sub.2 is realized.
FIG. 25 differs from FIG. 24 in that it is constituted to perform optical amplification in every wavelength to be multiplexed. That is, the light in wavelength .lambda..sub.1 is amplified by the optical amplifier 33a through the optical isolator 32c from the optical multi/demodulator 21c, and is fed into the optical multi/demodulator 21b through the optical isolator 32a, whereas the light in wavelength .lambda..sub.2 is amplified in the optical amplifier 33b through the optical isolator 32b from the optical multi/demodulator 21b, and is fed into the optical multi/demodulator 21c through the optical isolator 32d.
In such constitution, however, the devices of single functions such as band pass filter and optical multi/demodulator are composed so that the return light, if generated, passes through directly, and it is always required to assemble with optical isolators, and the number of parts increases in the system, and it takes time and labor in assembling.